Effect of oxidizers on hair dyes and hair melanin
Q. Dr. Said, I was wondering if you could give me a description of how oxidation opens the cuticle, disperses the melanin and binds the intermediates. As I have heard it has to do with the covalent bonds of the Oxygen and Hydrogen and the heat produced by the exchange of atoms. I have never had a chemist explain the process to me and I am very interested in knowing the specifics. Also, how are eumelanin and pheomelanin different from each other and is melanin ever gone completely and eliminated from the hair? Robin, San Diego, CA
A. Oxidation has only a minor role in opening up the cuticle. It is rather the alkaline base (in the form of ammonia or monoethanolamine) that causes swelling and loosening of the cuticle. Oxidation works progressively on melanin to bleach the color and causes irreversible changes in the melanin molecule and also to oxidize the dyes.
Dye intermediates undergo oxidation by the peroxide to form larger colored dye molecules. Dye intermediates by themselves are colorless. When a primary dye intermediate is oxidized, it loses hydrogen atoms and becomes activated so that it attacks a second dye intermediate in the form of a coupler or even another primary and binds to it covalently (strong bonding). This may be repeated more than once and the resulting molecule grows larger.
What you have heard about covalent bonds between hydrogen and oxygen, and about heat production is so general and vague that it does not help you understand the specifics of the chemical reactions involved. However don’t feel left out because you are not alone. Suffice to know that for a chemical reaction to take place, the products of a chemical reaction must achieve a lower energy level (or are more at ease) than the starting reactants which are (tense or edgy). It is like trying to push a rock downhill but before you are able to get it to the edge of the hill (where it can roll down on its own), it has to climb a small mound of dirt. In doing so you have to give it a push and spend some energy. In hair color, the push over the small mound of dirt that your rock has to overcome (or the energy to get your primary intermediate activated) is provided by the oxidizer removing hydrogen atoms from the primary intermediate and making it chemically active (grouchy ready to bite). Keep in mind, however, that some of the most useful interactions between hydrogen and oxygen are not covalent but rather weaker interactions known as hydrogen bonds which give water its fluid nature and vital role in life.
As far as the types of melanin, eumelanin and pheomelanin, both start out very similar during their synthesis in the cell. They are both polymers (chains) based on the amino acid tyrosine. Tyrosine is changed through the action of the enzyme “tyrosinase” to a new molecule called dopa, which is then changed by the same enzyme to dopaquinone (Albinos in general lack the enzyme tyrosinase and their cells cannot make melanin). At this point, eumelanin and pheomelanin go in different directions. Eumelanin chains continue growing by adding derivatives of tyrosine, while pheomelanins add derivatives of both tyrosine and the amine acid cysteine which contains sulfur. These cysteine derivatives form cross links or bridges among themselves which cause pheomelanins to be more tightly packed and resistant to bleach. The switch in the cell to synthesize either type of melanin (by adding tyrosine or adding cysteine) is related to genetic factors. Both types of melanins grow to form large granules of melanin chains wrapped around protein molecules. The chemical structure of both types is quite complex, and both have what is referred to as highly conjugated double bond structure which results in the dark color of the pigment and allows the molecule to absorb UV light and offer protection against harmful radiation. When melanin is oxidized, the conjugated double-bond structure starts to break up and the melanin gradually loses color and becomes “bleached”. It is not eliminated from the hair but remains in the hair in its colorless “bleached” form.